lEON SHIELDS FOR FORTS. 
3G9 
triangular gash, extending up to the side of the port-hole. 
Both the plate-upon-plate and the plank systems present, then, 
structural deficiencies ; and there can be, therefore, no excuses 
but economy, or facility of repairing, or of increasing the 
strength of the defence, for their adoption. All these reasons 
apply with greater force to the compound system of Mr. 
Hughes, as we shall hereafter show. It is with his system, then, 
that we shall now occupy our present thoughts. Timber 
backing jper se is next to useless as a directly resisting 
material ; but wood is highly useful in absorbing and deadening 
vibration. Hence a semi- elastic backing of wood and metal 
starts with a valuable inherent property. Every mechanic 
knows the value of a box -girder ; how the strains brought upon 
it in every direction are taken up equably by every portion of 
the hollow beam, and equally divided between parts in com- 
pression and parts in tension. Now, when Mr. Hughes’ hollow 
rails, or, as he calls them, stringers, are riveted up to the skin 
of the frame, anyone can see at a glance what a magnificent 
row of box-girders he has to take up and distribute the strains 
locally brought within such narrow limits upon the face-plate 
of the shield. These stringers infilled, and filled between, with 
strong teak constitute a semi-elastic cushion of enormous 
strength, and the greatest capacity for the widest distribution of 
the forces of blows or strains. This cushion can be supported 
by another in which the stringers cross each other at right 
angles. Instead of a weakness arising, strength accrues 
directly, for not only do the vertical stringers back up the 
horizontal at their intersections, but there is at every such 
place twice the amount of depth of metal to be cut av/ay, by a 
shot striking on those crossings, than there is upon the other 
best supported parts of the front plate. The holes cut in a 
plate for the bolts are always leading points of fracture for the 
shot, and the distention of the plate by shot is one of the main 
causes of the shearing of the bolts. Both these evils are 
avoided by the hollow stringer backing, because the bolts can 
pass between the stringers, and can screw up the face-plates 
direct as it were to the skin-plate, gripping in the semi-elastic 
backing as a pad between them. So far then, mechanically, 
every advantage is on the side of the hollow stringers as a pre- 
servative backing for the defensive structure. 
What as to its resistance to penetration ? To any but a me- 
chanical expert this backing would look like a sieve or a net full 
of meshes, through which the shot could squeeze its way. The 
9-inch shot has tried that, but the hollow stringers have gripped 
it hard and fast, would not let it go by; and then the 10-inch 
shot (400-pounder) has come up to help it — hit the pioneer, as 
it stuck, direct on the rear, drove it in an inch or two further. 
